Electrical stimulation can be advantageous over drug therapy because it can avoid the resulting side-effects. Further, patients who are refractory to, or non-compliant with, drug therapy may benefit from electrical stimulation. A method of treatment which includes providing peripheral stimulation in combination with drug therapy may allow for therapy benefit to be obtained with lower dosages or may allow some patients to obtain sufficient benefit from the combination therapy (while this does not occur when the drug is provided without peripheral stimulation), or may otherwise provide greater symptom relief to the patient.
Transcutaneous Electrical Nerve Stimulation (TENS) methods and devices apply electrical currents to the skin in order to modulate underlying target tissue such as muscles and nerves. TENS systems deliver energy through a patient's skin with two or more surface electrodes that do not pierce the skin. This is considered less invasive than percutaneous electrical nerve stimulation (PENS) or implantable systems. TENS is commonly used in the treatment of, or amelioration of, symptoms related to acute and chronic pain. Diabetic peripheral neuropathy, and associated pain in the foot or lower leg, is an example of a disorder that may be treated by TENS of the lower leg. This site of pain is often proximate, or caudal, to the site of stimulation. However, TENS in the lower leg may, in turn, stimulate targets in the spine or brain, which may allow such benefit as treating lower back pain.
In TENS, electrodes are applied to a person's skin at a target area which is selected to allow the stimulation to produce desired effects. For treatment of pain or muscle stimulation the electrodes may be applied at candidate stimulation locations by an individual user or doctor until the symptom relief is obtained. Typically low current (generally less than 50-100 mA) and short duration pulses (e.g. 50-200 μsec) are applied at frequencies between about 1 and 200 Hz, although the frequencies in the 50 kHz range may also be used. The shape of the pulses can be monophasic or biphasic (e.g., charge balanced). In the case of PENS or magnetic stimulation, candidate stimulation site(s) can be similarly evaluated to assess a treatment site prior to treatment being provided.
Generic TENS devices usually include a pulse generator which connects to at least one set of 2 electrodes, each of which connected to the generator using wires. Additionally, a set may include 3 electrodes where 2 electrodes serve as anode and one electrode serves as cathode, or vice versa. The anode and cathode designations can be determined dynamically, and the assignment can instantaneously change during the provision of stimulation such as when using biphasic waveforms. Additionally, more than one pair or set of electrodes can be used and independently controlled by separate circuitry of the pulse generator. Generic TENS systems provide for flexibility, but may have disadvantages such as requiring some time and effort to set up correctly. Generic TENS systems may necessitate patient training and/or supervision by a doctor. There is a risk of electrodes being placed incorrectly.
Various disadvantages have been overcome by specialized TENS systems that are designed to treat disorders such as pain, migraine, tremor, high blood pressure, sleep, etc. These systems provide features such as: a) using electrodes provided at fixed inter-electrode distances and geometries by incorporating these into a fixed substrate or electrode array, b) using support structures designed to secure stimulation electrodes in place, and c) using software and hardware components that assist with providing therapy in a correct manner (e.g., ensuring the impedance is not too high before or during provision of therapy, automatically setting of stimulation parameters, etc.).
An example system is disclosed in patent U.S. Pat. No. 8,948,876 (incorporated by reference herein), entitled “Apparatus and method for relieving pain using transcutaneous electrical nerve stimulation” which is designed for the treatment of pain. Improved pain relief is provided using a TENS device is configured to assist with setting the stimulation intensity above an individual's sensory threshold. Nerve recruitment threshold is not disclosed and may be higher than this cutaneous sensory threshold.
In peripheral nerve stimulation therapy, the selection of an appropriate stimulation parameter value (e.g., amplitude) requires that stimulation is below an intensity level that causes moderate discomfort or pain. However, it is also critical that sufficient electrical activation of therapeutically-relevant nerve fibers is achieved in patients. For example, in the case of PENS or TENS stimulation of the posterior tibial nerve, the location and amplitude of stimulation is considered successful when the set up creates a motor evoked response (foot twitch or fanning out of the toes) is achieved. The “successful” stimulation parameters are different for different people and can be different for the same person on different stimulation sessions that occur at different times or days. This may also be true within the same stimulation session. Although a TENS device can be set to gradually and automatically increases stimulation intensity to a programmed target level that previously was found to be suitable for a patient, that level may not be good for subsequent treatments due to variation in patient sensitivity, or changes in electrode contact, impedance, and/or location. The clinical utility of regulating stimulation parameters, such as stimulus intensity, based on an electrical impedance signal of this approach is unclear. Various additional methods of assessing, or re-assessing, sufficient stimulation amplitude may thus be helpful in improving therapy.
Additionally, determining subjective sensory experience related to successful candidate sites or nerve recruitment thresholds may require feedback from a patient. In the case of PTN stimulation, successful sites and parameters may be assessed by motor responses in the foot. However, stimulation of SAFN has been found by the inventors not to be accompanied by this type of response. Instead a patient subjectively reports experiencing a sensation associated with nerve recruitment. When implantation is done using an anesthetic, intra or post-surgical assessment can possibly be made which includes patient assessment of the stimulation with respect to sensation. However, even local anesthetic may prevent accurate patient feedback in the peri-procedural window.
Accurate reporting of stimulation-evoked sensation may be difficult or impossible in some patients. This may be more challenging in older, diabetic, or cognitively impaired patients. In patients the SAFN may indeed be electrically activated, but some patients will not detect any sensation related to stimulation. Some patients may only experience a stimulation field adjacent to the electrode that fails to indicate proper nerve recruitment for nerves such as the SAFN. As a result, it may be difficult for a patient to provide accurate and reliable feedback that will facilitate the determination, selection or confirmation of therapeutically effective stimulation parameters and locations for electrode placement. Additionally, patient feedback may not allow robust selection between two candidate locations or protocols.
There is a need for a new and improved peripheral stimulation devices which address the issues and shortcomings associated with prior art devices, and especially in relation to provision of therapy for overactive bladder.